--[[
  file_picker.lua
  Cross-platform native file picker for LuaJIT (Windows + Linux).

  Windows: uses the native Win32 comdlg32.dll GetOpenFileNameA API via FFI.
           No external dependencies required.

  Linux:   there is no single native "file picker" API — shells out to
           whichever dialog tool is installed (zenity, kdialog, or yad).
           Install one of these:
             sudo apt install zenity      # GNOME/GTK
             sudo apt install kdialog     # KDE
             sudo apt install yad         # lightweight alternative

  Usage:
    local picker = require("file_picker")
    local path = picker.open_file{ title = "Select a file", filter = "*.txt" }
]]

local ffi = require("ffi")

local M = {}

-- ---------------------------------------------------------------------
-- Windows implementation
-- ---------------------------------------------------------------------
local function win_open_file(opts)
  opts = opts or {}

  ffi.cdef[[
    typedef struct {
      unsigned long  lStructSize;
      void*          hwndOwner;
      void*          hInstance;
      const char*    lpstrFilter;
      char*          lpstrCustomFilter;
      unsigned long  nMaxCustFilter;
      unsigned long  nFilterIndex;
      char*          lpstrFile;
      unsigned long  nMaxFile;
      char*          lpstrFileTitle;
      unsigned long  nMaxFileTitle;
      const char*    lpstrInitialDir;
      const char*    lpstrTitle;
      unsigned long  Flags;
      unsigned short nFileOffset;
      unsigned short nFileExtension;
      const char*    lpstrDefExt;
      long           lCustData;
      void*          lpfnHook;
      const char*    lpTemplateName;
      void*          pvReserved;
      unsigned long  dwReserved;
      unsigned long  FlagsEx;
    } OPENFILENAMEA;

    int GetOpenFileNameA(OPENFILENAMEA* lpofn);
    int GetSaveFileNameA(OPENFILENAMEA* lpofn);
  ]]

  local comdlg32 = ffi.load("comdlg32")

  local OFN_EXPLORER      = 0x00080000
  local OFN_FILEMUSTEXIST = 0x00001000
  local OFN_PATHMUSTEXIST = 0x00000800

  local MAX_PATH = 32768 -- generous buffer for long paths
  local file_buf = ffi.new("char[?]", MAX_PATH)

  -- Filter must be a "\0"-delimited, double-"\0"-terminated string,
  -- e.g. "Text Files\0*.txt\0All Files\0*.*\0\0"
  local filter = opts.filter or "All Files\0*.*\0\0"

  local ofn = ffi.new("OPENFILENAMEA")
  ofn.lStructSize = ffi.sizeof("OPENFILENAMEA")
  ofn.lpstrFile   = file_buf
  ofn.nMaxFile    = MAX_PATH
  ofn.lpstrFilter = filter
  ofn.lpstrTitle  = opts.title or "Select a file"
  ofn.Flags       = OFN_EXPLORER + OFN_FILEMUSTEXIST + OFN_PATHMUSTEXIST

  local picker_fn = opts.save and comdlg32.GetSaveFileNameA or comdlg32.GetOpenFileNameA
  local ok = picker_fn(ofn)

  if ok ~= 0 then
    return ffi.string(file_buf)
  end
  return nil -- user cancelled or an error occurred
end

-- ---------------------------------------------------------------------
-- Linux implementation
-- ---------------------------------------------------------------------
local function shell_escape(s)
  return "'" .. s:gsub("'", "'\\''") .. "'"
end

local function try_command(cmd)
  local handle = io.popen(cmd)
  if not handle then return nil end
  local result = handle:read("*l")
  local ok = handle:close()
  if ok and result and #result > 0 then
    return result
  end
  return nil
end

local function linux_open_file(opts)
  opts = opts or {}
  local title = shell_escape(opts.title or "Select a file")
  local save  = opts.save

  local candidates = {
    save
      and ("zenity --file-selection --save --confirm-overwrite --title=%s 2>/dev/null"):format(title)
      or  ("zenity --file-selection --title=%s 2>/dev/null"):format(title),
    save
      and ("kdialog --getsavefilename . --title %s 2>/dev/null"):format(title)
      or  ("kdialog --getopenfilename . --title %s 2>/dev/null"):format(title),
    save
      and ("yad --file-selection --save --title=%s 2>/dev/null"):format(title)
      or  ("yad --file-selection --title=%s 2>/dev/null"):format(title),
  }

  for _, cmd in ipairs(candidates) do
    local result = try_command(cmd)
    if result then return result end
  end

  return nil, "No supported dialog tool found (install zenity, kdialog, or yad)"
end

-- ---------------------------------------------------------------------
-- Public API
-- ---------------------------------------------------------------------

--- Opens a native file picker dialog.
-- @param opts table: { title = string, filter = string (Windows only), save = bool }
-- @return path (string) or nil, err
function M.open_file(opts)
  if ffi.os == "Windows" then
    return win_open_file(opts)
  elseif ffi.os == "Linux" then
    return linux_open_file(opts)
  else
    return nil, "Unsupported OS: " .. ffi.os
  end
end

return M